The present invention relates to a power steering device which is used to reduce the force required for operating the steering wheel of an automobile. More particularly, it relates to a power steering device which provides an appropriate steering sense, varying with the speed of the vehicle, the angle of steering, etc.
There is generally known a power steering device which is composed mainly of an oil pump for supplying a hydraulic fluid under pressure from an oil reservoir to the main body of the power steering device, a flow passage switching valve for controlling the distribution of the hydraulic fluid from the oil pump in accordance with the operation of the steering wheel, and a power cylinder having a piston which is movable selectively in either direction upon application of hydraulic fluid pressure in the right or left fluid chamber thereof with the movement of the piston transmitted to a pitman arm or the like to assist the steering effort. As the rotary motion of the steering wheel is used to switch the hydraulic circuit of the power cylinder, the flow passage switching valve is usually of the rotary type, composed of a rotor and a sleeve. The oil pump is usually of the vane type having a cam ring having a substantially oval cam surface and a rotor which is rotatable within the cam ring to move a vane radially inwardly or outwardly.
As the vane type oil pump is usually driven by the automobile engine, its output varies in proportion to the rotary speed of the engine. The capacity of the oil pump is so selected as to ensure a sufficiently large output for the operation of the power steering device, even during the rotation of the engine at low speed, so that the steering force may be reduced when the automobile is at a stop or running at a low speed. Accordingly, the output of the oil pump is higher during the rotation of the engine at a medium or high speed than what is required for the power steering device. Therefore, an orifice and a flow control valve, which is actuated by a difference between the pressure of the fluid upstream of the orifice and the pressure of the fluid downstream of the orifice, are usually provided in a hydraulic fluid supply line to return any excess of the fluid to the oil reservoir so that a constant supply of the hydraulic fluid is maintained.
The resistance or load to which the steering wheel is subjected, however, differs with the operating conditions of the vehicle, especially its speed. It does not coincide with the feel desired by the driver, however, even if the output of the steering device is controlled. That is, when the vehicle is at a stop or running at a low speed, a reduction in the supply of the hydraulic fluid disables the easy operation of the steering wheel. When the vehicle is running at a high speed, an increase in the supply of the hydraulic fluid makes the steering wheel sensitive to so light a touch that the driver may feel uneasy and turn the steering wheel an unnecessarily large extent, resulting in a lack of stability of the vehicle. It is difficult to achieve a properly balanced supply of the hydraulic fluid.
It has, therefore, been proposed to provide a hydraulic reaction force chamber in the rotary flow passage switching valve, as disclosed, for example, in Japanese Laid-Open Patent Application No. 52-133627. According to this arrangement, the hydraulic fluid is supplied to the reaction force chamber at a rate depending on, for example, the speed of the vehicle to impart to the steering wheel a reaction force which is proportional to the steering resistance so that the driver always has a proper sense of the steering force.
The supply of the hydraulic fluid to the reaction force chamber, however, requires a small auxiliary pump separate from or unitary with the usual oil pump, or the branching of a part of the fluid discharged by the oil pump. This not only complicates the construction of the device and its piping, but also creates a large loss of energy for driving the pump for the reaction force chamber. Moreover, there is no assurance that a proper sense of steering can always be obtained. The reaction force chamber requires as small a quantity of the hydraulic fluid as less than about one-tenth of the output of the main pump. It is, therefore, not practical to provide a special pump for supplying the hydraulic fluid to the reaction force chamber. The branching of a part of the hydraulic fluid from a line to the power cylinder also presents a number of problems. If a high hydraulic pressure is required for the reaction force chamber, the oil pump must provide an increased pressure, thus requiring an increased amount of energy. An increased cost is required for driving the device, and a rise in fluid temperature results in a reduction in the reliability.
It is particularly desirable for a power steering device to be designed so as to require a larger steering force to turn the steering wheel when the automobile is running at a high speed than when it is running at a low speed so that the driver may have a sense of steering that properly reflects the speed of the automobile, thereby ensuring improved stability in the operation of the automobile. It is also considered desirable to develop a device of simpler construction which satisfies the above requirement.